Wireless communication devices configured for operation in public land mobile networks (PLMNs), and certain other types of wireless communication systems perform an initial access procedure after powering on or when reactivating after an extended period of sleep. As a first step in the initial access procedure, the wireless communication device searches for and detects a synchronization signal that base stations or other access points in the network regularly broadcast.
The synchronization signals enable the wireless communication devices to align with the network in frequency and time, based on using the synchronization signals from a given access point to determine symbol boundaries in the network transmissions and thereby align their reception and transmission timing and frequencies with the network. A “symbol,” for example, comprises an Orthogonal Frequency Division Multiplex (OFDM) symbol, a pre-coded OFDM symbol, a single-carrier symbol, etc. However, the term “symbol” shall be given broad construction in this disclosure, because the teachings herein are not limited to a particular modulation scheme.
Such alignment in time and frequency is essential for subsequent communication. Example information about synchronization in existing systems, such as in networks based on the Long Term Evolution (LTE) standards, can be found in the following Third Generation Partnership Project (3GPP) Technical Specifications (TSs): 3GPP TS 36.211, version 11.2.0, 3GPP TS 36.212, and 3GPP TS 36.213, version 12.1.0.
Note that the 3GPP documentation refers to wireless communication devices as items of “user equipment,” where “UE” denotes a single wireless device and “UEs” denotes plural wireless devices. The term “wireless communication device” as used herein encompasses the term “UE” and more. Indeed, unless otherwise noted, the term encompasses essentially any type of wireless communication apparatus that is configured to communicate within a wireless communication network. Without limitation, then, the term “wireless communication device” encompasses smart phones, feature phones, cellular network modems and dongles, Machine Type Communication (MTC) or Machine-to-Machine (M2M) devices, along with wireless-enabled computers, laptops, tablets, and the like.
Wireless communication devices may also perform procedures similar to the initial-access synchronization when preparing for a handover between different coverage areas in the network—e.g., a cellular handover from one network cell to another network cell, within a cellular communication network. In such contexts, the wireless communication device may have a connection to a currently-serving cell, but may wish to evaluate reception conditions with respect to one or more neighboring cells. However, here, the network may provide assistance information to the wireless communication device, to reduce the time needed for acquiring neighboring-cell signals.
Conventionally, network radio nodes transmit synchronization signals using a few sector-wide beams, e.g., each covering up to 120 degrees of circular arc. These sector-wide synchronization-signal beams are transmitted essentially simultaneously and together cover the entire geographic zone or area that the radio node is intended to serve. As recognized herein, that approach to synchronization-signal transmission may be undesirable in future wireless communication systems.
For example, future communication systems are expected to make heavy use of high-gain narrow beamforming, to enable high-data-rate transmission coverage for distant users that could not be served at high data rates without the gain provided by beamforming. Providing these users with synchronization signals of sufficient received-signal quality also may require the use of beamforming. Further, at least some network implementations are expected to use grids of relatively narrow beams and it may not be possible in such systems to transmit beams having broad coverage within the overall service area.
As a further recognition herein, transmitting a synchronization signal at the same power in a wide beam over all directions may represent wasted energy. For example, the geographic coverage area surrounding a radio node may have an irregular shape because of obstructions or other geographic features in the area around the radio node. As a consequence, the maximum distance of users to be served from the radio node will not be uniform in all directions.
At least some of the above issues may be addressed by using a narrow, swept beam for synchronization signal transmission from a radio node. However, it is recognized herein that certain challenges arise when using a swept beam for synchronization signal transmission. For example, latency problems may arise as a consequence of wireless communication devices waiting for the beam to sweep through the azimuthal and/or vertical angles corresponding to their positions relative to the radio node. As another example, the utilization efficiency of radio resources by the radio node may be compromised by conventional approaches to swept-beam transmission of synchronization signals.